Death in Dark Glasses (Inspector Littlejohn)

Even the cleverest of criminals can be undone through no fault of their own. It was a foolproof scheme - on paper, and the only thing the murderer overlooked was the human element. After the death of his wife, Finloe Oates became a recluse and cut himself off from the world, so nobody thought it strange that they didn't see him. Nobody thought of making enquiries. Nobody ever would have done, if a runaway bank clerk hadn't started a chain of investigation, that once begun, grew to overwhelming proportions and soon became far too big for the local police to handle.